The chemistry of ionized Acetone/Ar mixtures under varying gas flow and mole ratio conditions: A matrix‐isolation study

The chemistry of ionized acetone : Ar mixtures under varying total gas flow rate and acetone : Ar mole ratio conditions has been studied using matrix‐isolation techniques. Gaseous acetone diluted in excess argon gas was subjected to electron bombardment with 300‐eV electrons. The products of subsequent reaction processes were matrix isolated and analyzed by Fourier transform infrared (FTIR) absorption spectroscopy. Products included 1‐propen‐2‐ol (the enol isomer of acetone), methane, ketene, carbon monoxide, ethane, ethene, acetylene and tricarbon monoxide. Variations in the total flow rate of gas resulted in changes in the efficiency of product formation without significant changes in the relative amounts of the major species formed. Variations in the acetone : Ar mole ratio at fixed total gas flow resulted in striking variations in the products formed, demonstrating a shift from single acetone molecule‐derived charge‐transfer ionization chemistry at low acetone mole ratios, to processes consistent with the participation of two or more acetone molecules at intermediate mole ratios. These results are interpreted in the context of ion‐molecule reaction processes, the onset of which occurs at intermediate acetone mole ratios. Ethane dehydrogenation products are proposed to result from product secondary ionization, a process that is prevalent at high ionizing electron fluxes. Copyright © 2008 John Wiley & Sons, Ltd.     

Ethane cation decomposition characterization by EBMI spectroscopy: gas‐phase dissociative recombination as a source of secondary products

The decomposition products of the d₆‐ethane cation following charge‐transfer ionization with Ar⁺, under conditions of varying ionization electron current, have been isolated in solid argon matrices at 18 K and examined using Fourier transform infrared spectroscopy. Gas samples containing 1 : 1600 d₆‐ethane : Ar were subjected to electron bombardment by using either a high (pin) or a low (plate) ionization density anode configuration with ionization currents between 20 and 150 μA. Under high ionization density conditions, the observed major products were d₄‐ethene (C₂D₄) and d₂‐acetylene (C₂D₂), with smaller yields of C₂D₅, C₂D₃, and C₂D. The yield of each dehydrogenation product was enhanced with increased current. Analogous experiments employing the low ionization density plate anode resulted in reduced C₂D₆ destruction and the formation of only C₂D₄ and C₂D₂. The results suggest the onset of dissociative recombination processes under high ion density conditions. In this context, the results can be interpreted as a dissociative recombination of primary ion products, which gives rise to further dehydrogenation, and appearance of additional neutral radical products. Copyright © 2012 John Wiley & Sons, Ltd.     

Photochemical Reactions and Photoinduced Electron‐Transfer Processes in Liquids,Frozen Solutions,and Proteins as Studied by Multifrequency Time‐Resolved EPR Spectroscopy

In this overview, modern multifrequency EPR spectroscopy, in particular at high magnetic fields, is shown to provide detailed information about structure, motional dynamics, and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise photochemical reactions in liquid solution and light‐initiated electron transfer processes both in biomimetic donor–acceptor model systems in frozen solution or liquid crystals and in natural photosynthetic‐reaction‐center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects. They contain valuable information about structure and dynamics of the transient reaction intermediates. Moreover, they are exploited for signal enhancement. Continuous‐wave (cw) and pulsed versions of time‐resolved high‐field EPR spectroscopy, such as cw‐transient‐EPR (TREPR) and pulsed‐electron‐spin‐echo (ESE) experiments, are compared with respect to their advantages and limitations for the specific system under study. For example, W‐band (95‐GHz) TREPR spectroscopy in conjunction with a continuous‐flow system for light‐generated short‐lived transient spin‐polarized radicals of organic photoinitiators in solution was performed with a time resolution of 10 ns. The increased Boltzmann polarization at high fields even allows detection of transient radicals without CIDEP effects. This enables one to determine initial radical polarization contributions as well as radical‐addition reaction constants. Another example of the power of combined X‐band and W‐band TREPR spectroscopy is given for the complex electron‐transfer and spin dynamics of covalently linked porphyrin–quinone as well as Watson–Crick base‐paired porphyrin–dinitrobenzene donor–acceptor biomimetic model systems. Furthermore, W‐band ESE experiments on the spin‐correlated coupled radical pair in reaction centers of the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge‐separated transient state. The results are compared with those of the ground‐state P₈₆₅Q_A. The high orientation selectivity of high‐field EPR provides single‐crystal‐like information even from disordered frozen‐solution samples. The examples given demonstrate that high‐field EPR adds substantially to the capability of ‘classical’ spectroscopic and diffraction techniques for determining structure–dynamics–function relations of biochemical systems, since transient intermediates can be observed in real time in their working states on biologically relevant time scales.     

Fourier transform infrared matrix-isolation analysis of acetaldehyde fragmentation products after charge exchange with Ar•+ under varied ionization density conditions

The products of the Ar^?+ charge exchange ionization of acetaldehyde have been isolated and compared with related photoionization results and computational work. Acetaldehyde has been used to assess the effect of varied ion density in the ionization region of the electron bombardment matrix isolation apparatus. The amount of acetaldehyde destruction has been measured for constant gas‐sample composition and constant ionization current for two anode geometries: a pin anode and a plate anode. For the same ionization current, a pin‐shaped anode demonstrates higher precursor molecule destruction efficiency (85%) than the plate‐shaped anode (30%), resulting in substantial effect on the yield and quantity of isolated products. When the plate anode is used, the observed infrared products correspond to matrix‐isolated carbon monoxide (CO), methane (CH₄), ketene (CH₂CO), ethynyloxy radical (HCCO), formyl radical (HCO^?), acetyl radical (CH₃CO^?), vinyl alcohol (H₂C = CH‐OH), and cationic proton‐bound dimer, Ar₂H⁺. When the pin anode is used, the same products are observed with different relative proportions and new absorption features corresponding to dicarbon monoxide (CCO) and methyl radical (CH₃^?) are observed. The surprising observation of infrared absorptions corresponding to vinyl alcohol along with low yield of products anticipated through the analysis of photoelectron–photoionization coincidence measurements suggests that the initially formed fragmentation products are able to further react within the matrix‐isolation environment to influence observed product yields. Related experiments, using the isotopomer CD₃CHO, suggest that the observed products are formed via radical–radical reactions that occur under the high pressure conditions of the matrix isolation environment. Copyright © 2011 John Wiley & Sons, Ltd.     

Transmetalation in the Suzuki–Miyaura Coupling: The Fork in the Trail

The Suzuki–Miyaura coupling is one of the few transition‐metal‐catalyzed C? C bond‐forming reactions that have been used in applications ranging from discovery chemistry to manufacturing processes. Although coupling proceeds through the generic three‐stage ‘oxidative addition, transmetalation, reductive elimination’ sequence, there are a number of features that differentiate the Suzuki–Miyaura process from other transition‐metal‐catalyzed cross‐couplings. Most of these features are centered around, or are a consequence of, activation of the boron reagent for transmetalation through one or both of two distinct pathways. This review focuses on the evidence that has been presented for this ‘fork in the trail′, and the potential to apply such mechanistic insight to the design of reaction conditions.     

Specific Processes and Scrambling in the Dehydrogenation of Ethane and the Degenerate Hydrogen Exchange in the Gas‐Phase Ion Chemistry of the Ni(C,H3,O)+/C2H6 Couple

A mechanistically unprecedented situation characterizes the gas‐phase ion chemistry of Ni(C,H₃,O)⁺ when reacted under thermal, single‐collision conditions with ethane. A dehydrogenation channel leading to Ni(C₃,H₇,O)⁺ is to 90% preceded by a complete loss of positional identity of all nine H‐atoms of the encounter complex (‘scrambling’), whereas ca. 10% of the reaction exhibit a selective C? H bond activation of the alkane. In addition, a degenerate H exchange between ethane and the (C,H₃,O) unit occurs as a side reaction, the mechanistic details of which remain unknown for the time being.     

Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

The group additivity method for Arrhenius parameters is applied to hydrogen addition to alkenes and alkynes and the reverse β‐scission reactions, an important family of reactions in thermal processes based on radical chemistry. A consistent set of group additive values for 33 groups is derived to calculate the activation energy and pre‐exponential factor for a broad range of hydrogen addition reactions. The group additive values are determined from CBS‐QB3 ab‐initio‐calculated rate coefficients. A mean factor of deviation of only two between CBS‐QB3 and experimental rate coefficients for seven reactions in the range 300–1000 K is found. Tunneling coefficients for these reactions were found to be significant below 400 K and a correlation accounting for tunneling is presented. Application of the obtained group additive values to predict the kinetics for a set of 11 additions and β‐scissions yields rate coefficients within a factor of 3.5 of the CBS‐QB3 results except for two β‐scissions with severe steric effects. The mean factor of deviation with respect to experimental rate coefficients of 2.0 shows that the group additive method with tunneling corrections can accurately predict the kinetics and is at least as accurate as the most commonly used density functional methods. The constructed group additive model can hence be applied to predict the kinetics of hydrogen radical additions for a broad range of unsaturated compounds.     

Visible‐Light‐Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis

Herein, we report a new visible‐light‐promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl–SCF₃ compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl–SCF₃‐containing cyclic ketone and oxindole derivatives can be accessed by radical‐polar crossover semi‐pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF₃ radical.     

Chemie freier cyclischer vicinaler Tricarbonyl‐Verbindungen (‘1,2,3‐Trione’). Teil 3

Chemistry of Free Cyclic Vicinal Tricarbonyl Compounds (‘1,2,3‐Triones’). Part 3. Polar and Redox Reactions of 1,2,3‐Triones with Enamines of Different Types – News on Oxonol Dyes, Radicals, and Biradicals The central C?O groups of cyclic 1,2,3‐triones possess outstanding electrophilic (electron‐pair‐accepting) as well as oxidizing (one‐electron‐accepting) properties. Thus, 1,2,3‐triones are chemically related to 1,2‐ and 1,4‐benzoquinones. Whereas polar reactions with carbanion‐like (electron rich) species give rise to nucleophilic addition reactions to C?O groups under exclusive C,C‐bond formation, SET (single‐electron transfer) or redox reactions effect a partial ‘carbonyl Umpolung’ via ketyl intermediates (C,C‐ and/or C,O‐bond formation). Here, we report on numerous reactions between electron‐rich, more‐ or less‐polar enamines with 5,5‐dimethylcyclohexane‐1,2,3‐trione ( 9a ) and 1H‐indene‐1,2,3‐trione ( 9b ). Various new derivatives of basic oxonol dyes were formed, including the first oxonol dye incorporating a 1,3‐dioxocyclohexyl moiety. A novel stable radical, 50 / 50′ , was obtained from 9b and 11a via addition, hydrolysis, and treatment with conc. H₂SO₄. Radical 50 / 50′ represents a vinylogous ‘monodehydroreductone’ and is, thus, related to monodehydroascorbic acid ( 143 ), to Russell's radical cation ( 144 ), to indigo ( 141 / 141′ ), and to quinhydrone.     

On the Role of the Electronic Structure of the Heteronuclear Oxide Cluster [Ga2Mg2O5].+ in the Thermal Activation of Methane and Ethane: An Unusual Doping Effect

The reactivity of the heteronuclear oxide cluster [Ga₂Mg₂O₅]^.+, bearing an unpaired electron at a bridging oxygen atom (O_b^.?), towards methane and ethane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). Hydrogen‐atom transfer (HAT) from both methane and ethane to the cluster ion is identified experimentally. The reaction mechanisms of these reactions are elucidated by state‐of‐the‐art quantum chemical calculations. The roles of spin density and charge distributions in HAT processes, as revealed by theory, not only deepen our mechanistic understanding of C? H bond activation but also provide important guidance for the rational design of catalysts by pointing to the particular role of doping effects.     

Copper‐mediated controlled radical polymerization in continuous flow processes: Synergy between polymer reaction engineering and innovative chemistry

Copper(0)‐mediated controlled radical polymerization (CRP), or single‐electron transfer‐living radical polymerization (SET‐LRP) is a robust and dynamic technique that has attracted considerable academic and industrial interest as a synthetic tool for novel value‐added materials. Although SET‐LRP possesses many advantages over other forms of CRP, this novel chemistry still requires concurrent engineering solutions for successful commercial application. In this highlight, the evolution of atom‐transfer radical polymerization chemistry and development in continuous processes is presented, leading to recent research on the use of SET‐LRP in continuous flow tubular reactors. The proofs of concept are reviewed, and remaining challenges and unexplored potential on the use of continuous flow processes with SET‐LRP as a powerful platform for the synthesis of novel polymeric materials are discussed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3081–3096     

On the Synthesis,Characterization and Reactivity of N‐Heteroaryl–Boryl Radicals,a New Radical Class Based on Five‐Membered Ring Ligands

The synthesis and physical characterization of a new class of N‐heterocycle–boryl radicals is presented, based on five membered ring ligands with a N(sp²) complexation site. These pyrazole–boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; B?H) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(B?H) by the fine tuning of the N‐heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O₂, oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N‐heterocycle–boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions.     

Dispersive solid‐phase extraction followed by vortex‐assisted dispersive liquid–liquid microextraction based on the solidification of a floating organic droplet for the determination of benzoylurea insecticides in soil and sewage sludge

A novel dispersive solid‐phase extraction combined with vortex‐assisted dispersive liquid–liquid microextraction based on solidification of floating organic droplet was developed for the determination of eight benzoylurea insecticides in soil and sewage sludge samples before high‐performance liquid chromatography with ultraviolet detection. The analytes were first extracted from the soil and sludge samples into acetone under optimized pretreatment conditions. Clean‐up of the extract was conducted by dispersive solid‐phase extraction using activated carbon as the sorbent. The vortex‐assisted dispersive liquid–liquid microextraction based on solidification of floating organic droplet procedure was performed by using 1‐undecanol with lower density than water as the extraction solvent, and the acetone contained in the solution also acted as dispersive solvent. Under the optimum conditions, the linearity of the method was in the range 2–500 ng/g with correlation coefficients (r) of 0.9993–0.9999. The limits of detection were in the range of 0.08–0.56 ng/g. The relative standard deviations varied from 2.16 to 6.26% (n = 5). The enrichment factors ranged from 104 to 118. The extraction recoveries ranged from 81.05 to 97.82% for all of the analytes. The good performance has demonstrated that the proposed methodology has a strong potential for application in the multiresidue analysis of complex matrices.     

Cu(0)/2,6‐bis(imino)pyridines catalyzed single‐electron transfer‐living radical polymerization of methyl methacrylate initiated with poly(vinylidene fluoride‐co‐chlorotrifluoroethylene)

A series of 2,6‐bis(imino)pyridines, as common ligands for late transition metal catalyst in ethylene coordination polymerization, were successfully employed in single‐electron transfer‐living radical polymerization (SET‐LRP) of methyl methacrylate (MMA) by using poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) (P(VDF‐co‐CTFE)) as macroinitiator with low concentration of copper catalyst under relative mild‐reaction conditions. Well‐controlled polymerization features were observed under varied reaction conditions including reaction temperature, catalyst concentration, as well as monomer amount in feed. The typical side reactions including the chain‐transfer reaction and dehydrochlorination reaction happened on P(VDF‐co‐CTFE) in atom‐transfer radical polymerization process were avoided in current system. The relationship between the catalytic activity and the chemical structure of 2,6‐bis(imino)pyridine ligands was investigated by comparing both the electrochemical properties of Cu(II)/2,6‐bis(imino)pyridine and the kinetic results of SET‐LRP of MMA catalyzed with different ligands. The substitute groups onto N‐binding sites with proper steric bulk and electron donating are desirable for both high‐propagation reaction rate and C? Cl bonds activation capability on P(VDF‐co‐CTFE). The catalytic activity of Cu(0)/2,6‐bis(imino)pyridines is comparable with Cu(0)/2,2′‐bipyridine under the consistent reaction conditions. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4378–4388     

Radical Alliances: Solutions and Opportunities for Organic Synthesis

The present account discusses in detail various mechanistic features of the degenerative radical addition‐transfer of xanthates and related thiocarbonylthio congeners and makes a comparison with the more classical Kharasch reactions to which it is similar in certain aspects. The xanthate group reacts reversibly with the ‘active’ radicals in the medium and is able to store them in a somewhat inactive form. This increases their effective lifetime in the medium and, at the same time, lowers their absolute concentration while regulating their relative concentration. These properties translate into a powerful carbon–carbon bond forming process, especially as regards intermolecular additions to electronically unbiased (‘unactivated’) alkenes. Most functional groups are tolerated, in particular polar functions that often require protection with other chemistries. This broad versatility is illustrated by examples where the xanthate addition to the alkene is combined with other, more classical reactions to provide a convergent, rapid access to a wide range of useful structures. Emphasis has been placed on the synthesis of open chain and more complex carbocycles, as well as on the transfer of chirality. These ‘radical alliances’ include organosilicon chemistry, the Diels–Alder cycloaddition and cheletropic extrusion of sulfur dioxide, the Claisen sigmatropic rearrangement, and the Horner–Wadsworth–Emmons (HWE) condensation.     

Multicyclic polyethers derived from 1,1,1‐tris(4‐hydroxyphenyl)ethane and 2,6‐dihalopyridines

Poly(pyridine ether)s were prepared in two ways: the polycondensation of silylated 1,1,1‐tris(4‐hydroxyphenyl)ethane (THPE) with 2,6‐difluoropyridine (method A) and the polycondensation of free THPE with 2,6‐dichloropyridine (method B). With method A, the THPE/difluoropyridine feed ratio was varied from 1.0:1.0 to 1.0:1.6. Cycles, bicycles, and multicycles were the main reaction products, and crosslinking was never observed. When ideal stoichiometry was used exclusively, multicycles free of functional groups were obtained. These multicycles were detectable in matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectra up to B₃₈C₇₆ with a mass of approximately 32,000 Da. With method B, the reaction conditions were varied at a fixed feed ratio to achieve an optimum for the preparation of multicyclic polyethers, but because of the lower reactivity of 2,6‐dichloropyridine, a quantitative conversion was not achieved. The reaction products were characterized with MALDI‐TOF mass spectrometry, viscosity measurements, and size exclusion chromatography. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5725–5735, 2004     

Palladium on magnetic Irish moss: A new nano‐biocatalyst for suzuki type cross‐coupling reactions

A novel heterogeneous magnetic palladium nano‐biocatalyst was designed by utilizing Irish moss, a family of sulfated polysaccharides extracted from algae, as a natural biopolymer. This magnetic Irish moss decorated with palladium (Pd–Fe₃O₄@IM) to form a biomagnetic catalytic system was synthesized and well characterized by FT–IR analysis, X‐ray powder diffraction, field emission scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, atomic absorption spectroscopy and transmission electron microscopy. The catalyst was stable to air and moisture and displayed high catalytic activity in ligand‐free Suzuki–Miyaura cross‐coupling reactions conducted under green chemistry reaction conditions. The aromatic ketones are produced by the cross‐coupling reaction between acid chlorides and aryl boronic acid derivatives in high yields.     

Photocatalyzed thiol–alkene coupling: Mechanistic study and polymer synthesis

Due to the “click” chemistry characteristics of the thiol–ene reaction, these transformations have been gaining an increasing amount of attention in current chemical research. The high efficiency and selectivity of these transformations have been useful for many areas of study, from small molecule organic synthesis, to polymer synthesis and functionalization, to bio‐conjugation reactions. In this work, a study of a novel method of photochemical thiol–ene reactions using alkyl halides and an tris[2‐phenylpyridinato‐C2,N]iridium(III) (Ir(ppy)₃) photocatalyst is investigated. This process is shown to progress rapidly and has the benefit of low catalyst and initiator concentrations relative to reagents as well as mild conditions associated with photochemical processes. To understand the mechanism of this process, catalyst and initiator concentrations and other reaction conditions are varied. To demonstrate the utility of this process, a step‐growth thiol–ene polymer is synthesized using dithiol and diene monomers and a crosslinked polymer network is synthesized as well. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1931–1937     

Oxidative and Photochemical Stability of Ionomers for Fuel‐Cell Membranes

To predict hydroxyl‐radical‐initiated degradation of new proton‐conducting polymer membranes based on sulfonated polyetherketones (PEK) and polysulfones (PSU), three nonfluorinated aromatics are chosen as model compounds for EPR experiments, aiming at the identification of products of HO^.‐radical reactions with these monomers. Photolysis of H₂O₂ was chosen as the source of HO^. radicals. To distinguish HO^.‐radical attack from direct photolysis of the monomers, experiments were carried out in the presence and absence of H₂O₂. A detailed investigation of the pH dependence was performed for 4,4′‐sulfonylbis[phenol] ( SBP ), bisphenol A (= 4,4′‐isopropylidenebis[phenol]; BPA ), and [1,1′‐biphenyl]‐4,4′‐diol ( BPD ). At pH ≥ pK_A of HO^. and H₂O₂, reactions between the model compounds and O₂^.? or ¹O₂ are the most probable ways to the phenoxy and ‘semiquinone’ radicals observed in this pH range in our EPR spectra. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings. Investigations at low pH are particularly relevant for understanding degradation in polymer‐electrolyte fuel cells (PEFCs). However, the chemistry depends strongly on pH, a fact that is highly significant in view of possible pH inhomogeneities in fuel cells at high currents. It is shown that also direct photolysis of the monomers leads to ‘semiquinone’‐type radicals. For SBP and BPA , this involves cleavage of a C? C bond.